Add isDevToolsOpen to public interface

[zenoamaro]

isDevToolsOpen is not present on a Window object since at least 0.13 (see, eg., #4487).

This patch adds isDevToolsOpen to a few places where it is missing.

You can test presence (or thereof) with a simple app:

<script>
  alert(nw.Window.get().showDevTools); // function...
  alert(nw.Window.get().isDevToolsOpen); // undefined
</script>

Issue was first encountered while testing on 0.14.0-sdk-1 from npm after upgrading from ^0.12.

[rogerwang]

Thanks but it should come with an implementation.

[zenoamaro]

Thanks for checking out.

As far as I can see, the implementation is there already, just not exported to the public interface, hence the patch.